Semiconductor memory is widely used in various electronic devices such as mobile computing devices, mobile phones, solid-state drives, digital cameras, personal digital assistants, medical electronics, servers, and non-mobile computing devices. Semiconductor memory may include non-volatile memory or volatile memory. A non-volatile memory device allows information to be stored or retained even when the non-volatile memory device is not connected to a power source.
One example of non-volatile memory uses memory cells that include reversible resistance-switching memory elements that may be set to either a low resistance state or a high resistance state. The memory cells may be individually connected between first and second conductors (e.g., a bit line electrode and a word line electrode). The state of such a memory cell is typically changed by proper voltages being placed on the first and second conductors.
Some reversible resistance-switching memory elements may be in the high resistance state when first fabricated. The term “FORMING” is used to describe putting the reversible resistance-switching memory elements into a lower resistance state for the first time after fabrication. After a FORMING operation is performed, the reversible resistance-switching memory elements may be reversibly switched between a high resistance state and a low resistance state.
One theory that is used to explain the FORMING mechanism, as well as the reversible resistance-switching mechanism is that one or more conductive filaments are formed by the application of a voltage to the reversible resistance-switching memory elements. One example of a reversible resistance-switching memory element includes a metal oxide as the reversible resistance-switching memory material disposed between the first and second conductors.
In response to a suitable voltage between the first and second conductors, one or more conductive filaments form in the metal oxide, resulting in one or more conductive paths between the first and second conductors of the reversible resistance-switching memory element. The conductive filaments lower the resistance of the reversible resistance-switching memory element. Application of another voltage between the first and second conductors ruptures the conductive filaments, thereby increasing the resistance of the reversible resistance-switching memory element. Application of still another voltage between the first and second conductors repairs the rupture in the conductive filaments, once again decreasing the resistance of the reversible resistance-switching memory element.
The initial formation of the conductive filaments may be referred to as “FORMING.” The rupture of the filaments may be referred to as RESETTING, which puts the reversible resistance-switching memory element in a high resistance (RESET) state. The repair of the rupture of the filaments may be referred to as SETTING, which puts the reversible resistance-switching memory element in a low resistance (SET) state. After completing the FORMING process, the reversible resistance-switching memory element may be repeatedly switched between the SET and RESET states by repeatedly RESETTING and SETTING the reversible resistance-switching memory element. Data values may then be assigned to the high resistance RESET state and the low resistance SET state.
The FORMING process may impact the ability of the reversible resistance-switching memory element to exhibit proper switching behavior over time. For example, the reversible resistance-switching memory element may switch consistently between the high resistance state and the low resistance state in response to appropriate voltages, which may be referred to as “switching within the intended window.”